Porcelain enamels are non-combustible materials obtained by baking a vitreous substance onto a surface of a steel sheet, and have various properties such as heat resistance, weather resistance, resistance to chemicals, water resistance, stain resistance and the like as well as a beautiful surface appearance. As to the steel sheet material used for porcelain enameling, it is required to have firing strain property, fishscale resistance, enamel adhesion property, resistance to blistering and pinhole defects and the like as well as press formability in accordance with use or purpose, among which properties the enamel adhesion property is particularly important together with the fishscale resistance.
Heretofore, decarburization capped steels have been used as a steel sheet for porcelain enameling satisfying the above properties. Such decarburization capped steels are produced by subjecting a slab after ingot making-blooming step to hot rolling and cold rolling and then subjecting the resulting sheet to decarburization and denitration through open-coil annealing process. However, continuous operations such as continuous casting, continuous annealing and the like are used for energy-saving and cost reduction in the existing steel-making process, so that the production method and steps using the above decarburization capped steel become a past process.
When steel sheets having excellent fishscale resistance, enamel adhesion property and press formability are produced by the above continuous casting method, extra-low carbon steels having a C content of not more than 0.005 wt % (hereinafter shown by % simply) are used as a starting material and combined with Ti or B as disclosed, for example, in Japanese Patent laid open No. 61-276958 and Japanese Patent Application Publication No. 54-3446. The thus continuously cast steel sheets for porcelain enameling are almost treated in such a manner that they are pickled with sulfuric acid in a weight reduction through pickling of 20-100 g/m.sup.2 so as to form uneven portions of about 1 .mu.m on the surface of the steel sheet or further subjected to Ni flashing to precipitate metallic Ni on the top of the convex portion formed through the pickling in an amount of 0.1-5.0 g/m.sup.2, and then subjected to a single enameling in which an undercoat glaze (ground coat) containing about 0.5% of each of NiO and CoO as an oxide for promotion of adhesion property is applied and fired or a double enameling may be applied as in FIG. 3 in which a topcoat glaze (cover coat) is applied after the application of ground. These have a good adhesion property {PEI adhesion index&gt;80%: PEI [adhesion test method (ASTM: C313-59) recommended by P.E.I. (Enameling Associate in USA)]}.
In the existing enameling makers, an enameling method is provided wherein only alkali degreasing is conducted as a pretreating operation and an undercoat glaze (ground coat) containing about 1.0% of each of NiO and CoO as an oxide for the promotion of adhesion property is directly applied and fired without pickling with sulfuric acid and Ni flashing, as shown in FIG. 2. That is, this method is advantageous to the enameling maker because the pretreating step is omitted and the disposal of waste liquid from the pretreating liquid is useless to bring about the large reduction of the cost.
However, when adopting the enameling method using this type of the glaze, the continuously cast steel sheets for porcelain enameling have still a problem that the adhesion property is considerably poor as compared with the decarburization capped steel. Particularly, even when the good adhesion property is exhibited at the stage of single undercoat enameling, the remarkable degradation of the adhesion property is caused at the stage of double enameling.